Vane-type pumps are conventionally employed in aircraft turbine engine fuel supply systems to obtain sufficient fuel pressure at the engine during low-speed starting conditions. Problems arise, however, because of low vapor pressure and volatility of aircraft fuels, and because of air bubbles trapped in the fuel supply system. Pressure at the fuel tank is determined by the surrounding atmosphere at the altitude of the aircraft. Pressure must be lowered at the pump inlet to cause fuel flow. However, the limit to which pump inlet pressure can be lowered is the true vapor pressure of the fuel. When the inlet pressure equals the true vapor pressure, the fluid boils --i.e., is converted from liquid phase to gas phase. When this occurs at the inlets of conventional pumps, fuel flow stops.
It is thus necessary that a fuel pump system for aircraft engine applications be capable of handling fuel in two phases at the pump inlet, while being able to operate at true vapor pressure of the inlet fuel. Various devices have been proposed for separately boosting fuel pressure well above true vapor pressure prior to the primary fuel pump stage. However, these devices sometimes permit air bubbles in the fuel to enter the primary or high-pressure pumping stage, resulting in cavitation damage when present for extended periods of time.
It is therefore a general object of the present invention to provide a rotary hydraulic vane pump that is adapted to handle two phases of fluid flow at the pump inlet while scavaging the inlet line to substantially reduce or eliminate fuel vapors and air bubbles that may arrest fuel flow and/or cause cavitation damage at subsequent stages of the fuel supply system. Another object of the present invention is to provide a hydraulic pump of the described character that is adapted to operate at a vapor/liquid inlet ratio up to 1.0 without cavitation. A further object of the invention is to provided a boost pump for an aircraft turbine engine fuel supply system that is of economical and efficient construction in terms of the stringent weight and volume requirements in aircraft applications, and that provides reliable service over an extended operating lifetime.